sysroot: Load bootloader configs via boot_fd

sysroot: Load bootloader configs via boot_fd

This was a general principle cleanup, preparation
for handling VFAT for /boot for systemd-boot/BLS
support.

However I ran into an ugly corner case in our
unit tests that pointed at a sysroot without a
boot directory. The previous logic handled
ENOENT for boot/loader but not /boot.
Continue to cope with that degenerate situation.

Signed-off-by: Colin Walters <walters@verbum.org>